That's an incredibly interesting point!
Indeed, most of the times, we use "congiuntivo" (subjunctive tense) after the word "che".
However, with verbs like "pensare" (to think), "immaginare" (to imagine) and "credere" (to believe) you could use the "condizionale" (conditional tense) when the sentence states something possible.
Paolo pensa che potresti andare via. (conditional tense) ---> Paolo thinks that you could go away as there is the actual possibility that you could go away
Paolo pensa che tu possa andare via. (subjunctive tense) ---> Paolo thinks that you could go away, but it's just his opinion
However, the actual sentence in the conversation is slightly different:
Pensavo che potremmo fare una festa in giardino
(I) thought that (we) could have a party in the garden
In this specific case, it is not possible to use the subjunctive tense as the speaker is making a proposal, stating that "they could have a party". In these cases, when you want to refer to what could or should happen, you need to use the conditional tense.
Hope this helps, let me know if you have any other questions!